The geology and geomorphology of the Gesäuse

Geologie im NationalparkThe Gesäuse is largely composed of limestone and dolomite rock from the primordial ocean. Ice age glaciers shaped the landscape, giving the national park its current form. Even today, the geological history is clear to see for anyone hiking or climbing in the area.

The Gesäuse – Part of the Limestone Alps

Varying in width between 30 and 50 km, the Northern Limestone Alps range from the Alpine Rhine in the west to the Vienna Basin in the east. The rocks that make up the Limestone Alps were formed some 250 million years ago in the shallow waters of the primordial ocean known as Tethys. 190 million years ago in Jurassic times, the continent Pangaea began to break apart forming the separate continents we know today and the large oceans in between. Around 90 million years ago, Africa began moving to its current location causing the Mediterranean Sea to shrink and previously submerged layers to rise. The huge forces at work caused parts of the Limestone Alps to overlap each other in thick strata, whereas at other points the original substratum, the greywacke or slate zone, broke through.

The current geological situation is well-illustrated on the map compiled by Otto Ampferer (1935).

Limestone and Dolomite

Limestone and dolomite are the most common types of rock found in the Limestone Alps. While dolomite falls apart quite easily, limestone is a solid rock and forms the dramatic peaks of the Ennstal Alps, which rise to over 2,000 m and are therefore part of the Limestone High Alps. The mountains to the north are mostly composed of dolomite. They have few rock faces and are therefore counted as belonging to the Limestone Pre-Alps. More striking still is the contrast found on the southern edge of the Ennstal Alps, which is characterised by the gentle mountains of the Greywacke Zone, which is largely comprised of shale deposits. Originally it was made up of clay, lime and volcanic rock formed during the Palaeozoic era in the period between 450 and 375 million years ago. Greywacke is rich in iron ore deposits and the Erzberg open-pit mine in Eisenerz in Styria is of course a prime example.

Limestone is composed of the remains of marine organisms. This is revealed in the fossilized bivalve molluscs (megalodonts) whose thick shells are clearly visible even today. In the Gesäuse, ancient limestone banks are found in the Buchstein and the Hochtor Group. The Admont Reichenstein, on the other hand, was formed from a reef and smooth surfaces reveal the calcified remains of algae, sea sponges and coral. The dolomite typical of the Johnsbach valley was also formed from algal skeletons but is much more brittle than the hardened lime sludge. Beneath it is a layer originating from the time when the sea level once sank and the clay sediments and sand from the rivers prevailed over the organic lime. Today, this ribbon of what is known as Raibler strata runs through the entire Gesäuse mountains and separates the Wetterstein Dolomite from the Principal Dolomite.

Cut and Groove

Some 850,000 years ago the ice age began in the Eastern Alps. In the Gesäuse we only see traces of the last cold period, which started 120,000 years ago and lasted until the end of the ice age 12,000 years ago. From a huge glacial region, which covered most of western Austria, a glacier ran through the Ennstal valley all the way to the entrance of the Gesäuse near Weng. Here the ice built up and spread out over the mountain pass of the Buchauer Sattel. Glaciers also formed in the Gesäuse itself, starting at the cirques such as Roßschweif, Sulzkar, Tellersack or Hinterwinkel. Global warming at the end of the ice age 12,000 years ago caused many of the glaciers in the Alps to melt, among them the Enns Glacier and other glaciers in the Gesäuse region. The sheer weight and slow movement of the ice cut through and grooved out the Enns Valley to such an extent that it was left 200 m deeper upstream of the Gesäuse than it is today. It later filled up with water and the clay, gravel and sand deposited in this large lake caused it to silt up, forming the basis for the upland moors of the Enns Valley near Wörschach, Pürgschachen and Frauenberg.

Today

The processes that led to the forming of the Gesäuse of course continue today - the slow uplift and erosion caused by the River Enns, for example. Frost and ice split the rock. Scree bears witness to the effects of the invisible forces of nature on the rocks. Water infiltrates limestone quickly, forming large cave systems within the mountain. This leads to severe drought in the mountain area but also causes water springs to form in other areas, which are essential for the wellbeing of animals and plants in the Gesäuse.

For more information please visit our Geology exhibition in Gstatterboden.